In recent years, in the automobile industry, it has become an urgent problem to develop a vehicle structure capable of reducing injuries to a passenger at a time of collision. On the other hand, reduction in weight of the vehicle body is also important for improving fuel efficiency. For solving these problems, application of materials with higher strength, high-strength steel sheets as steel materials in particular, is considered. However, generally it is said that increase in strength leads to deterioration in formability. For expanding application, it is important to improve formability, particularly, stretch flange formability.
For solving such problems, development of a material with excellent stretch flange formability is in progress. For example, in Patent Document 1, there is disclosed a material with stretch flange formability improved by controlling microstructure such as ferrite and bainite. Further, in Patent Document 2, there are disclosed aluminum alloy sheets with excellent stretch flange formability by defining plastic anisotropy and uniform elongation in a tensile test in a specific direction.
However, formability in an actual part is determined not only by material properties, but is affected by die configuration, lubricating conditions, forming conditions, and/or the like in a complicated manner. Therefore, it is necessary to set these complicated factors appropriately, together with materials, so as to take advantage of excellent material properties. For such purposes, numerical analysis techniques are applied.
In Patent Document 3, there is disclosed a method of predicting a fracture or a wrinkle, which is a forming defect at the time of formation, using a finite element method. According to this method, analysis is performed with the finite element method, and generation of fracture or wrinkle is determined using data of strain and/or stress of an element of interest. However, when using such a method, it is required to perform element discretization by an appropriate size according to the analysis target. When analysis is performed with inappropriate element discretization, there is a fear that the prediction results in over or under estimation and hence does not correspond to the reality.    [Patent Document 1] Japanese Patent Application Laid-open No. 2002-60898    [Patent Document 2] Japanese Patent Application Laid-open No. 2006-257506    [Patent Document 3] Japanese Patent Application Laid-open No. Hei 8-339396